[FLINK-11657][tests] Fix deprecated API compatibility issue for WordCountMapreduceITCase.

[wenhuitang]

What is the purpose of the change

This pull request fixed deprecated API compatibility issue for WordCountMapreduceITCase.

Brief change log

WordCountMapreduceITCase tested deprecated API, but the related APIs have been removed , so this pull request removed the useless code.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

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[yanghua]

LGTM

[zentol]

There was no reason for this to be a separate PR from #7771, it's effectively the same issue.